Optical inspection allows for rapid and effective identification of defects in semiconductor objects, which include (but are not limited to) semiconductor wafers, reticles, mask patterns, and other items that are the result of or are used in fabrication of miniaturized electronic devices.
Various illumination and imaging systems have been proposed for optical inspection tools. For example, some systems use lamps or lasers to illuminate the object under inspection, with a detector or an array of detectors used to image areas of interest on the object. The detector output can be analyzed in any number of ways to determine whether a defect exists in the area. However, a semiconductor object oftentimes will comprise areas of different types.
As an example, FIG. 1A shows an example of a wafer die 110 comprising a periphery 112 at the die edges, with an array area 114 in the interior. Wafer die 110 may comprise one of many dies formed on a semiconductor wafer. Periphery 112 may represent logic and I/O circuitry, while array area 114 comprises memory. It should be noted that other dies may have other distributions and types of area, and so this discussion is for purposes of example only.
FIG. 1B shows an array 116 of frames. Some (but not all) inspection tools may logically divide a die or other area of a semiconductor object into a plurality of areas for inspection purposes. “Frames” are one example of such areas. Generally, a “frame” may be a unit imaging area of a tool. For two-dimensional detectors, a frame will comprise an area imaged at a given time (by one or more detectors), while for one-dimensional or point detectors, a frame will comprise cumulative acquired data over a given time period.
In this particular example, array 116 comprises several rectangular frames of the same size and shape. Other tools may use frames of different shape, number, and/or configuration. FIG. 1C shows array 116 overlaid on die 110.
Some inspection tools may comprise one or more detectors, and may inspect an object on a frame-by-frame basis. For example, a tool may image one or more frames of the object and then change its view of the object to image additional frames of different parts of the object. For example, the object may be moved while the tool components remain stationary, the object may remain stationary while some tool components are repositioned, and/or both the position of the object and tool may be adjusted.
Regardless of the underlying inspection methodology, however, the different properties of areas of a semiconductor object may lead to difficulties in inspection. For example, FIGS. 2A and 2B show an example of signal diagrams representing dark-field illumination as reflected from a part of a wafer containing an array and periphery, with defects in both areas. Each diagram also includes an indicator of the saturation limit of the detector, which represents the upper boundary of the detector's dynamic range.
Diagram 120 of FIG. 2A shows the case of illumination that is sufficient to illuminate a defect in the array area, with the defect represented by the signal variance indicated at 124. The defect in the periphery area is represented at 126. In FIG. 2A, the periphery defect 126 (and other periphery signal) is above the saturation limit 122 of the detector. This is because the intensity of reflected light from the periphery area is much higher than the intensity of the reflected light from the array area. In a typical wafer, the ratio between the intensities may reach orders of magnitude.
Diagram 128 of FIG. 2B shows the case of illumination that is set to bring a periphery defect 132 into the dynamic range of the detector (i.e. so that signals from the periphery are below the saturation limit 122). In this case, though, the signal of defect 130 in the array area is much smaller. Thus, the array defect may be difficult or impossible to detect.
Accordingly, there remains a need to provide for optical inspections using sufficient light to view defects without sacrificing inspection quality for other defects.